The embodiments described herein relate generally to transmission of power from a plurality of remotely located power generation systems, and more specifically, to direct current (DC) transmission of power from the power generation systems to a remote location via a superconducting DC cable transmission system.
In distributed generation applications, the site of power generation is remote from the available electric grid or load point and bulk power is often transmitted over long distances. In an off-shore wind farm, for example, power generated by individual wind turbine generators typically is processed by power electronic converters to convert a variable voltage, variable frequency output to a fixed voltage, fixed frequency output. The individual wind turbine generators operate at different speeds and therefore the frequency of the output of each generator may be different. When an alternating current (AC) transmission system is used to transmit the generated power to the on-shore electric grid, the outputs from the individual generators must be synchronized to the utility network frequency before applying the power to the electric grid. The power generated from the turbines is then brought together by a collection system that includes transformers and switchgears for isolating individual turbines and stepping up the voltages, usually to tens of kilovolts. The collection system is cabled to an off-shore substation that increases the voltage further, usually to hundreds of kilovolts (kV). It is then transmitted through subsea cable to an on-shore substation, where it is coupled to the on-shore electric grid through isolating switch-gears and transformers.
For applications where bulk power is transmitted over long distances, conventional high voltage alternating current (HVAC) transmission provides technical challenges. Furthermore, HVAC transmission lines are inconvenient for use in densely populated areas and are not an efficient solution for off-shore wind farms where subsea cables must be used for power transmission. Capacitance causes charging current to flow along the length of an AC cable. Because the AC cable must carry this current as well as the useful source current, this physical limitation reduces the source carrying capability of the AC cable. Because capacitance is distributed along the entire length of the cable, longer lengths result in higher capacitance and higher resulting charging current. To transmit the charging current and the useful source current, the AC cables must be over-rated, which increases the cost of the AC cable. As the cable system design voltage is increased to minimize the line losses and voltage drop, the charging current also increases.
DC transmission can be achieved more efficiently over longer distances than AC transmission. Medium voltage (MV) or high voltage (HV) DC transmission typically requires power electronic converters which are capable of converting between HVAC and HVDC. Power generation systems that utilize DC transmission typically include a plurality of AC to DC converters that are coupled in parallel and voltage controlled. The voltage level of the power transmitted over the DC cable is maintained substantially constant, while a current level varies depending on the power output of the plurality of generators. If a fault occurs within the voltage controlled system, the voltage level is maintained substantially constant, while the current may rapidly increase. Although DC collection and transmission systems have several advantages over AC systems, voltage controlled DC transmission systems are most commonly used in military and research applications because expensive switchgear is needed to perform interrupt functions due to high short circuit current in a parallel DC topology.
Furthermore, high voltages are typically utilized in high power transmission, for example, in the range of hundreds of kilovolts (kV) to transmit hundreds of megawatts (MW) of power, since high current power transmission is less efficient than high voltage power transmission. Typical, HVDC power conversion is expensive and complex, especially at high power levels because of the level of transmission voltage needed for high power transmission. For example, to not exceed acceptable transmission losses for a bulk power transmission of, for example, 300 MW, a transmission voltage of upwards of 200 kV may be required.
High temperature superconducting (HTS) cables are available for AC power transmission and for DC power transmission. HTS cables have high conductivity, and therefore, low transmission losses, which reduces a transmission voltage level needed for long distance power transmission. HTS AC cables are expensive relative to the cost of HTS DC cables. Furthermore, HTS DC cables configured to transmit power having a current that varies are more complex and expensive than HTS DC cables configured to transmit power having substantially constant current. A varying current causes a changing magnetic field within a HTS DC cable, which creates eddy current losses and reduces power transmission efficiency. In a voltage controlled DC transmission system, the current may vary enough to prevent efficient power transmission using an HTS DC cable.